During the fabrication of electronic devices, integrated circuit devices are encapsulated in a protective package for ease of handling and protection from environmental or mechanical damage. The highest degree of protection is provided by a hermetic package design. Hermetic packages are usually glass sealed to achieve both environmental isolation and electrically isolated leads.
In a conventional ceramic dual-in-line package, a low temperature sealing glass is used to seal a metal (typically Alloy 42 or Kovar) leadframe between ceramic components. The hermetically sealed package encases an integrated circuit device which is attached to one of the components and electrically connected to the leadframe.
Hermetic glass sealed metal package are disclosed in U.S. Pat. No. 4,410,927 by Butt with copper or copper alloy components and in U.S. Pat. No. 5,023,398 to Mahulikar with aluminum alloy components.
The packages are heated and cooled several times during assembly and operation. For a reliable package, the sealing glass must have a coefficient of thermal expansion (CTE) close to that of the package components. Generally, the variation in CTE should not be in excess of 10%. Absent this match, thermally induced stresses are applied to the glass to metal or glass to ceramic seals and fracture is likely.
Low temperature solder glasses are used for package fabrication. The sealing temperature should be below about 450.degree. C. to prevent thermal degradation of the integrated circuit device during glass sealing. Suitable glasses are based on a lead borate, lead zinc borate, lead borosilicate or lead-zinc borosilicate matrix. Additions are made to the glass matrix to provide the desired sealing characteristics, corrosion resistance, electrical resistivity, dielectric constant, etc. These glasses generally have a (CTE) of from 80 to 130.times.10.sup.-7 in/in/.degree. C.
This CTE range is generally unacceptable for sealing either ceramic packages or metal packages. Ceramic packages, typically alumina Al.sub.2 O.sub.3 based, have a CTE of about 65.times.10.sup.-7 in/in/.degree. C. Copper and copper alloys have a CTE of about 170.times.10.sup.-7 in/in/.degree. C., while aluminum and aluminum alloys have a CTE of about 270.times.10.sup.-7 in/in/.degree. C.
To overcome the CTE mismatch, a filler may be added to the glass. The filler is selected to have a coefficient of thermal expansion different than the glass matrix and the resulting composite has an intermediate coefficient of thermal expansion. The coefficient of thermal expansion of the composite is closely related to the volume fraction of filler and glass matrix in the sealing glass composite.
Fillers to lower the CTE of a glass are disclosed in U.S. Pat. No. 3,954,486 to Francel et al and include silica, zirconia, quartz and beta-eucryptite. U.S. Pat. No. 3,963,505 to Dumesnil et al discloses the use of zinc oxide while U.S. Pat. No. 4,006,028 to Nofziger discloses a mixture of a coarse and fine refractorys such as zirconium silicate (ZrSiO.sub.4).
Fillers to raise the coefficient of thermal expansion include calcium fluoride and barium fluoride as disclosed in the above noted patents to Smith III. Silicon dioxide coated calcium fluoride as a filler is disclosed in U.S. Pat. No. 4,185,139 to Smernos.
A suitable filler has limited solubility in the glass matrix. Only the undissolved volume fraction of the filler influences the CTE of the composite. Coating the calcium fluoride particles with silicon dioxide, while effective is both costly and time consuming. A preferred alternative is uncoated particles.
The CTE of the electronic package components may be vastly different than that of the glass matrix. Large quantities of filler, in excess of 30-50 volume percent, may be required for the composite to achieve an acceptable CTE. The particle morphology of conventional fillers is randomly shaped and frequently angular so that particle distribution obtained by sieving is large. When the particles are in close proximity within the glass, they interlock deteriorating the flow characteristics of the glass.